Backwashable pressure filter units of the above-mentioned kind have long been used. They have the advantage of additional filter surface area as compared to filter units containing only a single filter tube. However, and particularly when handling difficult process liquids, there has been as tendency for prior plural tube filter units to progressively clog their filter tubes by a gradual building up of solids on the outside thereof despite periodic backwashing. Further, there has been observed a tendency to more rapid buildup of clogging solids toward the upper end of the filter tubes and in the upper part of the space closely surrounded by the several filter tubes within the filter casing.
In attacking these problems, Applicant has inserted into each of the filter tubes a spiral diffuser in the form of aC generally rigid twisted plate, or ribbon, preferably of stainless steel, so as to try to distribute the outflow of backwash liquid more evenly along the length of the filter tubes. Spiral diffusers have previously been known, for example from U.S. Pat. No. 3,256,678 (Bertin) for use in separation of gases by diffusion through a porous wall, and from U.S. Pat. No. 3,648,754 for use in reverse osmosis and hyperfiltration channels.
However, the mere insertion of spiral diffusers into the filter tubes did not eliminate the tendency to build up solids over a plurality of filtration-backwash cycles.
In dealing with this problem, Applicant recognized that much of the time in industrial installations it is difficult to obtain an adequate water supply for backwashing. In a typical filter unit containing three filter tubes, simultaneous backwashing of the filter tubes may require approximately 250 gallons of water per minute for even partially effective backwashing. This backwash liquid supply may be needed for only one or two minutes every four to six hours, particularly when a newly cleaned, or new, filter unit is first put in service. It is difficult to justify such a large water supply capability for such infrequent use since it requires large pumps, large piping and other correspondingly large equipment elements, as well as high energy input, all of which make a backwash water supply of this kind expensive to install and operate, particularly where in difficult filtration applications the filter unit may eventually clog and require disassembly and manual cleaning, a very expensive process in terms of lost operating time and labor expense.
It has been known to individually and in sequence backwash plural filter tubes in a single casing by means rotatable on the central axis of the casing and extending radially therefrom to engage a circular array of filter tubes. An example is Reece U.S. Pat. No. 3,703,465 (assigned to the assignee of the present invention) in which synchronously rotating backwash input and receiving arms respectively engage the open end of a filter tube and the opposite end of a subcasing thereof for a subinterval of a backwash period before rotating onto the next filter tube and subcasing in the circular array thereof. Since each filter unit has its own subcasing, this apparatus is more equivalent to a plurality of filter units each with a single filter tube. Hersey U.S. Pat. No. 2,731,107 provides a different example in which a hollow backwash arm is supported central to the casing for rotation therein and has a backwash liquid outlet which slides circumferentially along the circular array of filter tube ends to engage same one at a time. The apparatus disclosed by Hersey is for gas filtering not for pressure liquid filtering.
Prior rotating arm-type backwash systems are not suitable for compact pressure liquid filter units in which plural (for example three) filter tubes are close spaced within a single compact casing, since there is not adequate room for central location of rotational support and drive means for the rotating backwash arm nor for additional mechanism that may be required for limiting scuffing of the rotating backwash arm against the ends of the filter tubes. It may be quite impossible to apply the rotating arm-type backwash apparatus as an improvement to an existing compact filter unit, particularly to a close packed array thereof, simply because of inadequate space, and aside from questions of mechanical complexity, wear, mechanical limitations against backwashing of a filter tube out of sequence, etc.
According to the present invention, cleaning by backwashing can be made much more effective, to keep the filter unit operating in a clean or substantially clean condition over a large number of filtration-backwash cycles, so as to avoid frequent removal from service, tear-down and manual cleaning occurrences, even with significantly reduced backwash liquid flows. For example, under the present invention operation over many filtration-backwash cycles with effective backwashing and without significant degradation in filtration time has been achieved even with a reduction in water flow rate in the magnitude of from 250 gallons down to about 190 gallons per minute. Thus, it has been found possible under the present invention to minimize backwash liquid piping size, pump horsepower and other related installation and operating expense factors, and yet still provide improved backwash cleaning, and substantially reduce tendency toward solids buildup over multiple filtration-backwash cycles.
Accordingly, the objects and purposes of the present invention include provision of:
a backwash apparatus for a multi tube filter unit capable of operating at reduced backwash liquid flow rates and yet substantially reducing the tendency toward solids buildup and clogging, particularly in the area between adjacent faces of close spaced filter tubes and spaces in the casing closely surrounded by a plurality of filter tubes; and
an apparatus as aforesaid which does not require increase in the diameter of the filter unit and is capable of new or replacement use in a wide variety of filtering applications, such as paper mills, oil refineries, and so forth.
Other objects and purposes of the invention will be apparent to persons acquainted with apparatus of this general kind upon reading the following description and inspecting the accompanying drawings.
The objects and purposes of the invention are met by providing a multi-element filter unit including a backwash apparatus comprising a filter casing with provision for alternative process liquid flow therethrough and backwash liquid flow in the opposite direction therethrough. Plural filter tubes are close spaced within the filter casing. A flange engages the filter tubes and divides the filter casing into first and second chambers. Apparatus for sequentially backwashing the filter tubes during a backwash interval removes collected solids from between the filter tubes by flow of backwash liquid from one filter tube outward toward the exterior of the adjacent filter tubes. This backwash apparatus includes poppet units opposed to the open ends of the filter tubes and selectively actuable to close the opposed ends of ones of the filter tubes while leaving another open to backwash flow therethrough, such that each filter tube is backwashed in sequence.